Silicones are widely employed in the electrical and electronics industries due to their unique properties, which include low alpha particle emissions, very good moisture resistance, excellent electrical insulation, excellent thermal stability, and very high ionic purity. In particular, silicone encapsulants are used to improve the reliability of electronic devices by providing protection against environmental moisture, UV radiation, ozone, and weathering. Moreover, recent advances in semiconductor packaging, namely, the development of chip scale or chip size packages, have created a critical demand for high performance silicone encapsulants.
Curable silicone compositions comprising a polydimethysiloxane polymer and an organopolysiloxane resin are well known in the art. These compositions typically employ up to about 50 parts by weight of resin per 100 parts by weight of polymer. Furthermore, several cure systems are recognized, including hydrosilylation, peroxide, moisture, condensation, and radiation.
However, silicone elastomers produced from such compositions typically exhibit several fold increases in coefficient of thermal expansion and modulus in the temperature range corresponding to the amorphous-crystalline phase transition, typically from -50 to 40.degree. C., rendering them unsuitable for use as encapsulants in chip scale packages at very low temperatures. As the amorphous-crystalline transition is traversed during thermal cycling, stresses develop in the chip scale package due to thermal expansion and contraction of the silicone encapsulant and the difference in coefficients of thermal expansion between the substrate and silicon die. Thermally-induced stresses can damage critical components of a semiconductor package, including solder joints and lead wires, resulting in poor reliability.
Inorganic fillers such as silica and alumina are typically added to silicone compositions to reduce the coefficient of thermal expansion of cured products. Such fillers have very low coefficients of thermal expansion compared to a silicone elastomer and thus produce an overall reduction in thermal expansion of the elastomer. However, fillers also increase the viscosity of a silicone composition and, consequently, often reduce processability.
Various approaches to controlling the moduli of silicone compositions have been reported in the literature. For example, nonfunctional silicone oils have been employed to regulate the moduli of elastomers produced from curable silicone compositions. Unfortunately, silicone oils have a tendency to undergo post cure migration, causing shrinkage of the composition and contamination of surrounding materials.
Reactive silicone oils have also been used in silicone compositions to lower the modulus of a cured silicone rubber. For example, U.S. Pat. No. 5,216,104 to Okami et al. discloses an addition-type curable silicone rubber composition comprising (A) an organopolysiloxane containing at least two alkenyl groups per molecule, (B) an organopolysiloxane containing an average of one alkenyl group per molecule, (C) an organohydrogenpolysiloxane containing at least two silicon-bonded hydrogen atoms per molecule, (D) a platinum catalyst, and optionally an organopolysiloxane resin. The '104 patent teaches that the silicone rubber composition can give a low modulus silicone rubber by curing which neither contaminates the material in contact therewith nor deforms owing to shrinkage, without hindering the adhesion of the composition during its curing stage. Unlike the '104 patent, the present invention does not require an organopolysiloxane containing an average of one alkenyl group per molecule. Also, the '104 patent does not teach the molecular weight limitations on the organopolysiloxane resin, resin to polymer ratio, and adhesion promoter of the present invention.
Organopolysiloxane resins have also been used in silicone compositions to regulate the moduli of cured materials. For example, U.S. Pat. No. 5,705,587 to Hatanaka et al. teaches a room temperature-curable silicone elastomer composition comprising (A) a diorganopolysiloxane composed of (a) a silanol-enblocked diorganopolysiloxane and (b) a diorganopolysiloxane that is endblocked at one molecular chain terminal by silanol and at the other terminal by trialkylsiloxy; (B) an alkoxy functional silane; (C) an organopolysiloxane resin; (D) a curing catalyst; and (E) a diorganopolysiloxane containing at least two carboxyl groups. The '587 patent also teaches that the aforementioned composition has an excellent pre-cure workability and cures into a low-modulus, high-elongation silicone elastomer. However, the '587 patent does not teach the polydiorganosiloxane polymer, organopolysiloxane resin, resin to polymer ratio, organohydrogenpolysiloxane, and hydrosilylation catalyst of the present invention.
U.S. Pat. No. 5,373,078 to Juen et al. discloses curable organosiloxane compositions comprising a liquid polyorganosiloxane having an average of at least two alkenyl groups per molecule, (B) a liquid organohydrogenpolysiloxane having at least three silicon-bonded hydrogen atoms per molecule, (C) a hydrosilylation catalyst, and (D) a liquid MQ organosiloxane resin. The '078 patent also teaches that the liquid MQ resin reduces the viscosity of the curable compositions and the moduli of cured elastomers prepared from the compositions without any substantial adverse affect on the physical properties of the cured elastomers. However, the '078 patent does not teach the organopolysiloxane resin, resin to polymer ratio, and adhesion promoter of the present invention.
EP 0767216 to Schmidt discloses moisture-cure and addition-cure silicone resin/fluid alloy compositions that exhibit high strength and toughness over a predetermined service temperature range comprising (A) an organosilicone resin, (B) a predominately linear silicone fluid, and (C) a predetermined quantity of a crosslinker having at least three functional groups per molecule. The EP 0767216 patent teaches that the crosslinked product of (A) and (B), when free of an inorganic filler, exhibit a shear loss modulus (G") greater than or equal to 10.sup.6 Pa and a loss tangent greater than or equal to 0.10. However, the EP0767216 application further discloses that such compositions have utility in the manufacture of molded and extruded articles and, therefore, does not teach the adhesion promoter of the present invention.
U.S. Pat. No. 5,756,598 to Chung et al. discloses an organosiloxane composition curable to a silicone elastomer comprising (A) a polydiorganosiloxane having an average of at least two silicon-bonded alkenyl groups per molecule; (B) an organohydrogensiloxane having an average of at least two silicon-bonded hydrogen atoms per molecule; (C) a platinum group catalyst, and (D) a resinous organosiloxane copolymer having an average of at least two alkenyl groups per molecule. The '598 patent also teaches that the aforementioned composition is curable to a silicone elastomer having improved weep properties. However, the '598 patent does not teach the resin to polymer ratio of the present invention.
The present inventors have discovered that a silicone composition comprising a critical concentration of an organopolysiloxane resin having a number-average molecular weight within a specified range cures to form an elastomer having excellent adhesion to a variety of substrates and a very low coefficient of thermal expansion and storage modulus over a wide temperature range.